chart disregards the gradualness of the adjustment to the new 5MX and the waves that would take place as the adjustment occurs. Its point is that the effect of both a change in 5MQ and in 5M5S will be a rise in the population, but the population curve then resumes in the same direction as before with the AM55 , while it changes its angle with the AM0.
Even if all deaths between 55 and 100 years of age were to be eliminated the ultimate rate of increase of the population would not be affected. The population curve would turn upwards during the next two or three generations, but would then resume the same geometric sequence that it would follow if the present birth and death rates continued without change. Applying this in practical terms, the conquest of heart disease would not alter the ultimate rate of increase of the population; the advent of antisepsis to eliminate puerperal infection did have a clear permanent effect on the rate of increase.
The effect of a change in an age-specific birth rate on the Net Reproduction Rate is the change multiplied by Lx :
Thus a birth change is of more consequence when it occurs to a young age group than to an old, but the difference in this framework is trifling with modem mortality. The survivorship SLXIIQ sloped downward from 4.86 at age 15 to 19 to about 4.63 at 45 to 49 for United States women in 1966. Hence a decrease of the birth rate to women 15 to 19 would lower the Net Reproduction Rate only 5 percent more than the same decrease to women 40 to 44. But we will find that for rapidly growing populations the true impact on growth of what the women around their 20's do is fully twice as large as the impact of what the women in their 40's do. In this important respect the Net Reproduction Rate is an inadequate measure.
The Turnover of Generations
A comparison of Canada and the United States for 1967 shows that the Net Reproduction Rate does not tell the whole story of replacement. Canada's NRR was slightly higher than that of the United States, 1.216 against 1.205. The Canadian girl child just born could expect to bear slightly more girl children than the United States girl child at 1967 rates. But the Canadian would marry older and have her children later within marriage; her length of generation— a measure something like the average age of childbearing-was 27.3 years against the United States 26.2 years. This means a more rapid turnover of generations in the United States.
If the length of generation is T years, and the rate of growth compounded momently is /•, then the ratio of increase over a year is er, and the ratio ofon Rate and even a Net Reproduction Rate of more than unity.ife or lives saved may be supposed the result of a durable improvement in medical technique, then we want to know the effect on populations of theage by 2.42 years in comparison with Sweden 1800—evidently the survivorship of Honduras was espe-ion Change on the Attainment of Educa-bout 20 percent. For the less developedHilton Salhanick has observed that some women practicing the rhythm method will break or lose their thermometers at the critical juncture in theirright, therefore alwayshas some c